1911 Encyclopædia Britannica/Bajocian
|←Baize|| 1911 Encyclopædia Britannica, Volume 3
|See also Bajocian on Wikipedia, and our 1911 Encyclopædia Britannica disclaimer.|
BAJOCIAN, in geology, the name proposed in 1849 by d'Orbigny for the rocks of Middle Jurassic age which are well developed in the neighbourhood of Bayeux, Calvados. The Bajocian stage is practically equivalent to the Inferior Oolite of British geologists. It corresponds fairly closely with the Lower and Middle Brown Jura of Quenstedt, and with the Dogger of Oppel. By means of the fossil ammonites the Bajocia strata have been subdivided into the following zones, in descending order:—
|Zone of||Parkinsonia Parkinsoni and Cosmoceras garantianum|
|"||Coeloceras subcoronatum (Humphriesianum)|
|"||" Murchisonae||} =||Substage Aalénien|
|"||" opalinum||of Mayer-Eymar.|
It should be remarked that some European geologists prefer to include the Parkinsonia zone in the base of the overlying Bathonian (q.v.).
The Bajocian rocks of Europe are mostly limestones of various kinds, very frequently oolitic. At Bayeux, the type district, they are ferruginous oolites; in the Jura and Lorraine a coral limestone overlies a crinoidal variety; calcareous sandy and marly beds occur in Maine and Anjou; in Poitou the limestone is dolomitic and bears nodules of chert. Rocks of the same age, as recognized by their fossil contents, have a wide range; they are found in north Africa, Goa, Somaliland, German East Africa, and north-west Madagascar; through southern Europe they may be followed into Turkestan, and the Kota-Maleri beds of the Upper Gondwana series of India may possibly belong to this stage. In South America they appear in Bolivia, Chile and Argentina; in North America, in British Columbia, Dakota, Mexico, Oregon and California. The Bajocian sea also included parts of New South Wales, New Zealand (Flag Hills beds?), Borneo and Japan, and it extended into the polar region of eastern Greenland and Franz Josef Land.
In addition to the ammonites already mentioned, the large belemnites (Megateuthis giganteus) and terebratulas (T. perovalis) are worthy of notice; crinoids and corals were abundant, and so also were certain forms of Trigonia (T. costata), Pleurotomaria and Cidaris.
See Jurassic; also A. de Lapparent, Traité de géologie, vol. ii. (5th ed., 1906); and H. B. Woodward, "The Jurassic Rocks of Britain," vol. iv., 1894 (Mem. Geol. Survey); both works contain references to original papers.
- (J. A. H.)